A tungsten catalyst is used as a catalyst for various oxidation reactions using hydrogen peroxide as an oxidizing agent. The oxidation reactions are usually conducted in the presence of water, and the tungsten catalyst used is dissolved in a reaction mixture. Therefore, various methods for recovering tungsten contained in the tungsten catalyst used from the reaction mixture are reported.
For example, JP 8-291104 A and JP 55-4459 B disclose methods for recovering tungstic acid by contacting an oxidation reaction mixture with an ion-exchanged resin. JP 46-41526 B discloses a method comprising precipitating tungsten catalyst by adding acetone, tetrahydrofuran, dioxane, n-propanol or isopropanol in an amount of eight times by weight or more to an oxidation reaction mixture and recovering it. WO 2005/110962 A discloses a method comprising cooling the reaction mixture obtained by reacting wastewater which contains hydroxycaproic acid and which is discharged from the oxidation reaction of cyclohexane with hydrogen peroxide in the presence of a tungsten catalyst and isolating solids of tungstic acid precipitated.